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Tasks that run for extended periods of time should provide a notification mechanism to alert upper layers when they terminate abnormally. Failure to do this does not cause any resource leaks because the threads in the pool are still recycled, however, it makes failure diagnosis extremely difficult. 

The best way to handle exceptions at a global level is to use an exception handler. The handler can perform diagnostic actions, clean-up and shutdown the JVM or simply log the details of the failure. This guideline may be violated if the code for all tasks has been audited to ensure that no exceptions are possible. Nonetheless, it is usually a good practice to install a handler to initiate recovery. Other ways include overriding {{ThreadPoolExecutor}}'s {{afterExecute()}} hook. This hook is called in the event of a successful execution (completion of {{run()}}) or an exception (as opposed to an error). Similarly, the {{terminated()}} hook is called after the tasks finish executing. It can be overridden to release resources acquired by the thread pool over time, much like a {{finally}} block.

h2. Noncompliant Code Example

This noncompliant code example consists of class {{PoolService}} that encapsulate a thread pool and a runnable class, {{Task}}. The {{run()}} method of the task can throw runtime exceptions such as the {{NullPointerException}}. 

{code:bgColor=#FFCCCC}
final class PoolService {
  private final ExecutorService pool = Executors.newFixedThreadPool(10);
		  
  public void doSomething() throws InterruptedException, IOException {	   
    pool.execute(new Task());
  }	
}

final class Task implements Runnable {
  @Override public void run() {
    // ...
    throw new NullPointerException();
    // ...	
  }
}
{code}

final class PoolService {
  // The values have been hard-coded for brevity
  ExecutorService pool = new CustomThreadPoolExecutor(
      10, 10, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(10));
  // ...
}

class CustomThreadPoolExecutor extends ThreadPoolExecutor {
  // ... Constructor ...
  public CustomThreadPoolExecutor(
      int corePoolSize, int maximumPoolSize, long keepAliveTime, 
      TimeUnit unit, BlockingQueue<Runnable> workQueue) { 
    super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
  }


  @Override
  public void afterExecute(Runnable r, Throwable t) {
    super.afterExecute(r, t);
    if (t != null task does not notify upper layers when it terminates unexpectedly because of the runtime exception. Moreover, there is no recovery code.

h2. Compliant Solution (Custom exception reporter)

This compliant solution refactors the task so that it catches {{Throwable}} and forwards it to a custom exception reporter (see [EXC01-J. Use a class dedicated to reporting exceptions] for details on class {{MyExceptionReporter}}).

{code:bgColor=#ccccff}
// ...

final class Task implements Runnable {
  @Override public void run() {
    try {
      // ...
      throw new NullPointerException();
      // ...
    } catch(Throwable t) {
      // Exception Executeoccurred, anyforward recoveryto codehandler
    }
  MyExceptionReporter.report(t);	 
    // ... Perform task-specific cleanup actions
  }

  @Override
  	
public void terminated() {
    super.terminated();
    // ... Perform final clean-up actions
  }
}

 }
}
{code}

{mc} overriding the afterExecute method in ThreadPoolExecutor}} is also suggested {mc}

h2. Compliant Solution (Uncaught exception handler)

This compliant solution sets an uncaught exception handler for the thread pool. During the construction of the thread pool, a {{ThreadFactory}} argument is passed. The factory is responsible for creating new threads and setting the uncaught exception handler on their behalf. The class {{Task}} remains the same as the noncompliant code example.

{code:bgColor=#ccccff}
final class PoolService {
  private static final ThreadFactory factory =
   new
   new ExceptionThreadFactory(new MyExceptionHandler());
  private static final ExecutorService pool =
      Executors.newFixedThreadPool(10, factory);

  public void doSomething() throws InterruptedException, IOException {	   
    pool.execute(new Task()); // Task is a runnable class	      	    	      
  }
 
  public static class ExceptionThreadFactory implements ThreadFactory  {			
    private static final ThreadFactory defaultFactory =
        Executors.defaultThreadFactory();
    private final Thread.UncaughtExceptionHandler handler;

    public ExceptionThreadFactory(
        Thread.UncaughtExceptionHandler handler)
    {
      this.handler = handler;
    }

    @Override public Thread newThread(Runnable run) {
      Thread thread = defaultFactory.newThread(run);
      thread.setUncaughtExceptionHandler(handler);
      return thread;
    }
  }
	  
  public static class MyExceptionHandler extends ExceptionReporter
  
    implements Thread.UncaughtExceptionHandler {

    private final Logger logger = Logger.getLogger("com.organization.Log");
    // ... 

    @Override public void uncaughtException(Thread thread, Throwable t) {
      logger.log(Level.SEVERE, "Thread exited with exception: " + thread.getName(), t);			      // Recovery or logging code
    }
  }	  
}
{code}

Note that the uncaught exception handler is not called if the method {{

final class PoolService {
  private final ExecutorService pool = Executors.newFixedThreadPool(10);

  public void doSomething() {
    Future<?> future = pool.submit(new Task());

    // ...

    try {
      future.get();
    } catch (InterruptedException e) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } catch (ExecutionException e) {
      Throwable exception = e.getCause();
      // Forward to exception reporter
    }
  }
}

 
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